Chert (Jasper/Chalcedony)

Physical Properties

Hardness: 6.5-7 (Mohs scale), Color: tan, brown, or pinkish-beige, Luster: waxy to dull, Crystal structure: cryptocrystalline, Cleavage: none, Fracture: conchoidal (shell-like curves), Specific gravity: 2.6

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or by the chemical replacement of existing rocks by silica-rich fluids in aquatic environments.

Uses & Applications

Historically used for stone tools and weapons due to its sharp edges when fractured. Currently used in aquarium gravel, garden decor, and occasionally as a semi-precious gemstone when polished.

Geological Facts

Chert is so hard it can strike sparks from steel, which led to its historical use as 'flint' in flintlock firearms. It is incredibly resistant to chemical weathering.

Field Identification & Locations

Identify in the field by its waxy texture and conchoidal fracture (curved breakages similar to thick glass). It cannot be scratched by a steel knife. Common in limestone regions or as rounded river cobbles.

Identified on: 4/17/2026

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